WP:Saving meta fields as post title

cpt.php

 <?php 
public function save_staff_title( $staff_title ) {

    $first = $_POST[META_PREFIX . 'first-name'];
    $last = $_POST[META_PREFIX . 'last-name'];
    if ( $_POST['post_type'] == 'staff' )
      $staff_title = $last . ', ' . $first;

    return $staff_title;
    
  }